Sorry if this guide is too raw, I just wrote it.

You can use this method with any amount of money.

Go to Runescape website > Community > Grand Exchange > Most Traded
In this page you will see 100 listed items from most traded to least traded. Any of the items in this page is good enough to use this method.

What you need to do basically is pick one or multiple items and buy 1 unit and sell 1 unit, by doing that you will get the "margin" of the price.

For example, I bought 1 unit of Gold Bar and sold 1 unit of Gold Bar, then I got those results in the Grand Exchange "Sales History" window:

Sold Gold Bar 550
Bought Gold Bar 500

This will give me a profit margin of 50gp per unit.

Another thing to consider is the amount allowed to be traded per 4hrs. (Yes, you are limited of buying X amount of items per 4 hours, different items have different limits)
Gold Bar for example have a limit of 10,000 units per 4hrs.
Some items have 25k, 10k, 5k, 500, 10, 2, 1. It depends, but most of the items in the "Most Traded" section have from 25k to 5k limit.

The amount limit is important because the profit you will get per Item you choose to trade is the result of the Limit minus one multiplied by the profit margin. (Gold Bar Limit = 10000 - 1, Profit Margin = 50gp. So the profit you will get by trading the full amount is 10000 - 1 x 50gp = 499,950gp)

What I would do next if I have a large amount of cash is setup a buy order for 9,999 units (1 less because you bought a unit to get the margin) at the price of 500gp (Yes, 500, not 501... I'll get into that later) resulting in a amount of 4,999,500gp.

It will take some time to finish the order but when it finish, get all the gold bars you bought and sell it for 550gp (Not 549, for the same reason Ill talk about down below) after it finish you will get 5,499,450gp.

You just had a profit of 499,950gp in 1 hours max, and that using only 1 slot of your Grand Exchange. If you are a F2P you got 3 slots, if you are a member you got 8 slots.

Doing that you can in a month get to 1M gp easily and after some time you can pay for membership using bonds, and it will take only a week to make ~50M to pay for it (Each bond is ~26M right now)

After some time doing that and getting rich you can adventure yourself in more expensive items like equipments and have huge margins of profit.

What I wanted to talk about up there when I was explaining about the margins and price you should set in your buy/sell orders is:
Please dont Undercut or Overcut anyone, if you found the margin to be 500 and 550, just buy at 500 or sell at 550 and wait for your order to fill, it wont take long... unless the price is Bullish(Rising) or Bearish(Falling) alot. This is important because if someone doesn't respect that which usually happens you guys will mess up your own margins and constantly having to find the margins again, losing time, item limit, gold and profit.

Btw I know this method is dependent on money, but it worked for me in the moments I didn't want to play the game and just wanted to chill or to read some book, watch some TV Show or whatever.

If you are beginning in the game you should use your skills to get some GP before trying to do that. But still, its possible, but you will be sitting at the GE for a long time to get a good amount of GP in one day.

If you have some Millions in your account it will be easier, you will be able to fully trade many items in the 4hrs limit. Making Millions of GP per day.